In common control type telephone systems, wherein the communication path between calling and called parties is set up by the common control portion of the equipment, it is of vital importance that any malfunction of the common control be quickly detected and identified, in order to minimize any resulting interruption in service. While malfunctions are usually readily noticeable during continuous use of the system, in some offices, such as those which provide telephone service for small communities, the use of the system may be reduced significantly during evening and early morning hours, and considerable periods of time may elapse between requests for service from the system and the attendant control functions carried out by the common control. A malfunction of the common control during these minimal use periods may continue unnoticed until traffic begins to pick up during the morning hours, with the result that, when a sudden demand for connection service from the system common control occurs, nothing yet has been done to correct the malfunction, and the service to be provided by the system must often be interrupted to detect and correct the problem at the time when the demand for service is high.
Unfortunately, the common control portion of the system is usually a hardwired special purpose computer, and the circuitry which would be necessary to test each portion of the common control would be prohibitively expensive, to say the least.